Spark plug



F. J. BOLL SPARK PLUG Jan. 6, 1942.

2 Sheets-Sheet 1 Filed y 20, 1940 llllllll INVENTOR 73177.]: 5041..

avx m ATTORNEY Jan. 6, 1942. BOLL 2,269,076

SPARK PLUG Filed May 20, 1940 2 Sheets-Sheet 2 INVENTOR FPA'DJ 5044. BY 7 {A ATTORNEY Patented Jan. 6, 1942 SPARK PLUG Fred J. Boll, Milwaukee, Wis., assignor to Globe- Union Inc., Milwaukee, Wis., a corporation of Delaware Application May 20, 1940, Serial No. 336,101

\ 15 Claims.

The present invention relates to an improvement in spark plugs for use with internal combustion engines.

Spark plugs of one type contemplated by the present invention comprise generally a metallic shell adapted to be, threaded into the cylinder head of the engine, an insulating body or spark plug barrel, usually of porcelain or ceramic material, fitted in the shell and hermetically sealed thereto by suitable gaskets, a center electrode which extends down through and is hermetically sealed in an axial opening in the barrel or insulating body, side or ground electrodes carried by the shell and cooperatively interrelated with the center electrode to define the spark gap or gaps, and a suitable binding post or terminal nut at the outer end of the center electrode to connect the plug in the ignition circuit.

For some time past there has been a trend in the design of spark plugs for internal combustion engines which has resulted in a decrease in the flash-over resistance between the terminal nut at the outer end of the center electrode and the gland nut, or between the center electrode and the metallic shell or housing, in structures where no gland nut is employed. The path which the flash-over current takes is along the outer surface of the insulating spark plug barrel.

And along with this decrease in flash-over resistance there has been an increase in the resistance of the spark gap circuit.

The decrease in the flash-over resistance is the direct result of the use of smaller spark plugs which has decreased the .height and also the diameter of the insulating spark plug barrel. The installation space available in current assembly engine designs places a definite limitation on the height of the insulator and on the peripheral dimensions of the hexagon formed at the upper end of the outer shell. This not only decreases the height of the insulator but reduces its outside diameter since the outside diameter of the insulator or spark plug barrel is limited by the inside diameter of the upper gasket, which in turn must be smaller than the dimensions of the hexagon since it is received in the structure of the upper end of the shell surrounded by the hexagon.

The increase in the resistance 'of the spark gap circuit is due to two causes. In the first place, because of the inherent shortcomings of certain combustion. chamber designs, such as stratification of the gaseous mixture, automotive engineers have been forced to the use of wider spark gaps. Secondly, the use of higher compression ratios and, higher compression pressures has further increased the resistance of the spark gap circuit, since with the same spark gap the resistance of the spark gap circuit is proportionate to the absolute pressure of the gaseous mixture between the electrodes.

And finally, the wear or erosion of the electrodes, and consequent increase in the width of the spark gap, is intensified in the modern automotive engine, due to the use of higher energy ignition systems and to higher combustion chamber temperatures.

As a result of these trends, the resistance of the spark gap circuit is becoming proportionately higher than flash-over resistance. Because of this unfavorable unbalance between the spark gap resistance and the flash-over resistance, the contamination of the outside surface of the spark plug barrel, resulting either from the accumulation of dirt or moisture thereon, plus the increase of the spark gap due to intensified wear, tends to shorten the effective life of a plug, in that upon the progress of these conditions to a certain point, the electric current, instead of following its intended path down the central electrode to the lower end thereof, and then across the spark gap to the side or ground electrodes, will follow the path of least resistance along the outside surface of the spark plug barrel. When this occurs, there is, of course, a failure of ignition.

Another cause of spark plug failure is the cracking or fracture of the insulating barrel of the plug which may result in the flow of the current from the center electrode through the crack, or break, directly to ground, thereby short-circuiting the spark gap. Cracking or fracture of the insulating barrels of spark plugs occurs all too frequently, and usually is caused by impact or by excessive stress to which the spark plug barrel is subjected as a result of careless handling of the plug, the misuse of 'tools in installing them, or the thermal shock occurring when the insulat ing barrel of a hot spark plug is sprayed or splashed with water or rain. The lowered mounting of automotive engines, and the elimination of splash pans in the modern automobile, increases the exposure of the spark plugs, and increases the likelihood of their being subjected to thermal shock.

An object of the invention is to provide a spark plug of standard, modern design, but having a protective insulating shield of special construction, combined with the spark plug in such a special way as to seal off the normal flash-over path from the terminal nut, along the surface of the insulating-barrel, to the gland nut or shell, and

so as to increase to a maximum the external flash-over path along the outer surface of the shield, thereby lessening the drop in flash-over resistance for the same degree of surface contamination per unit area, and insuring the discharge across the spark gap to produce the desired ignition.

Another object of the invention is to provide a shielded spark plug of this character, and having these advantages, and which is also efi'ective to protect the insulating barrel of the plug against thermal shocks, or against breakage, due to careless handling of the plug or the misuse of tools in installing.

Another object of the invention is to provide a spark plug having these advantages and capacities, and wherein the shield, so effectively combined with the plug, is to all intents and purposes a permanent part thereof and yet does not interfere with the installation or removal of the plug or disadvantageously increase its over-all dimensions.

A still further object of the invention is to provide a shielded spark. plug which not only has a long external fia'sh-over path of high resistance, but one which is so surfaced as to be repellent to moisture or condensation and resistant to the accumulation of dirt or foreign matter.

A further object of the invention is to provide a spark plug having these advantages and capacities, and which is simple and durable in construction, reliable and efiicient in operation, and easy and comparatively inexpensive to manufacture from materials and by means or facilities ordinarily available.

Other objects and advantages reside in certain novel features of the construction, arrangement, and combination of parts, which will be hereinafter more fully described and particularly pointed out in the appended claims, reference being had 'to the accompanying drawings forming a part of this specification, and in which:

Figure 1 is a view partly in side elevation. and partly in transverse, vertical cross section, showing a spark plug constructed in accordance with the present invention;

Figure 2 is a view in side elevation of plug shown in Figure 1;

Figure 3 is a view similar to Figure 1 showing a modified form of the invention;

Figure 4 is a view showing another modification of the invention;

Figure 5 is a view similar to views in Figures 1, 2, 3,and 4, showing still another form of the invention;'

Figure 6 is a view similar to Figure 5, showing another structural variant of the invention;

Figure 7 is a view partly in side elevation and partly in transverse vertical section, showing a slight modification of the form ofthe invention illustrated in Figure 3;

Figure 8 is a view in transverse, vertical section showing another embodiment of the invention, parts being shown in elevation for the sake of iithe spark lustration;

Figure 9 is a view similar to Figure 8, showing another modification of the type of structure illustrated in Figure 8; and

as at 2 so as to be adapted to be screwed into an internally threaded opening of a cylinder head of an internal combustion engine. Fitted in the metallic shell 9 is an insulating body or spark plug barrel. 3, usually constituted of porcelain, and hermetically sealed to the shell by means of the usual gaskets. In some instances the body is secured in the shell and the gaskets are compressed by means of a packing nut or gland ll, whereas in another type a portion of the outer end of the body is spun, crimped, or formed over to perform the function of the packing nut. As these particulars form no part of the present invention they need not be described in detail. A center electrode 5 extends down through an axial opening in the insulating body of the plug and is cemented or adhesively united thereto so as to be securely held in place and also be hermetically sealed to the insulating body. I'he inner end of the center electrode in eachinstance is cooperatively related to one or more side or ground electrodes, designated at 8. At the outer end of the center electrode a terminal nut l is provided to facilitate connection of the spark plug in the ignition circuit.

In all the forms of the present invention the insulating barrel 3 of the spark plug has combined therewith a protective jacket or shield, which is so interconnected and so interfitted with the barrel as to be, for all practical purposes, a permanent part thereof, and capable of function ing to seal oif the normal flash-over path from the terminal nut, along the surface of the insulating barrel, to the shell or gland nut, and, at the same time, of substantially lengthening or increasing the flash-over path along the external surface of the shield or jacket as well as resisting or repelling the accumulation of dirt, foreign matter, or moisture, and serving also the purpose of protecting the insulating barrel against the damaging efiects of impact and thermal shock.

In the forms of the invention shown in Figures 1 to 7 this protective jacket or shield takes the form of a pre-moulded or pre-formed sleevelike structure 8 of elastic insulating material, which is so tightly interfitted with the barrel as to be capable of performing the functions specified. Inorder to accomplish these purposes, the barrel 3 itself is specially constructed in that it has a series of parallel corrugations or annular ridges which extend circumferentially about the periphery of the barrel. The end corrugations,

Figure 10 is a view similar to Figures 8 and 9 of another structural variant of the type of spark plug illustrated in Figures 8 and In the embodiments .of the invention shown in Figures 1 to '7, inclusive, the spark plug comprises a metallic shell, designated generally at l, which has its lower end reduced and externally threaded upper end of the barrel 3.

designated at 9, may be larger than the intermediate corrugations E9. The elastic insulating sleeve 8 is moulded or pre-formed so that the internal opening thereof is smaller in diameter than the external diameter of the insulating barrel 3 of the spark plug, so that when the sleeve is assembled with the barrel it must be stretched or intensioned, and consequently will tightly engage the periphery of the barrel in close surface-to-surface contact therewith, with annular bands of the internal periphery of the sleeve engaged with the surfaces of the corrugations of the barrel at high unit pressure. The intimate contactthereby effected seals all the normal flash-over path along the outside surface of the barrel 3 and makes the insulating jacket of the sleeve a permanent part of the spark plug structure. The sleeve 8 has an integral inwardly directed flange II at its upper end which fits over the shoulder presented at the Itlis also recessed at its lower end, as at l2, to fit over the gland nut.

In these particulars the various forms of the .cumulation of contaminating matter.

invention shown in Figures 1 to 7, inclusive, are identical. Likewise, in the forms mentioned, the material, which may be used to constitute the sleeve-like structure, is the same. Quite a' wide variety of materials maybe utilized. It is essential only that the material when pre-formed or pre-rnoulded be elastic, have electrical and heat insulating properties, and preferably also be moisture or water repellent. Rubber may be employed, but preferably material which is synthetic in composition is used because it has been found to withstand the action of oil better than rubber. There are many synthetic rubber-like materials now manufactured which are capable of resisting the deteriorating effects of heat, mineral oil, and oxidation and any of them may be utilized.

The various forms of the invention shown in Figures 1 to 7 difier from each other in the external configuration of the protective sleeve. In the construction shown in Figures 1 and 2, the peripheral surface of the protective sleeve is in Figures 8 to 10, inclusive, the present invention proposes to provide a protective jacket or shield of rigid, plastic, moulded material, which is not only electric insulating and heat resistant, but possesses the other desirable properties. The materials contemplated are the various synthetic plastic materials, which are being currently rendered available, for example, the heat resistant phenolic resins, such as certain Bakelites. In the form of the invention shown in Figure 8, the spark plug comprises a metallic shell 20, in which the conventional insulating barrel 2| of porcelain is fitted and hermetically sealed by gaskets 22. Figure 8 illustrates the one-piece type of spark plug structure wherein the insulating barrel is held in place and the gaskets compressed by spincharacterized by the provision of, a series of parallel, concave, torroidal grooves l3, with sharp edges 14 between adjacent corrugations. This particular shape, especially when the sleeve iscoated with lacquer, hard wax, synthetic rubber paint, or the like, gives a long flash-over path and a high resistance'to water spray or condensation of atmospheric-moisture, and yet is well adapted to the practical limitations of synthetic rubber moulding methods. Many of the advantagesof the present invention, however, may be realized by constituting the surface or substantial portion of the sleeve with a series of rounded corrugations l5 of the form shown in Figure 3, or constituting a substantial portion of the surface of a series of sine wave corrugations l6, as illustrated in Figure 7. The maximum increase in flash-over path along the external surface of the sleeve may be had by providing the periphery thereof with a series of sharp V- shaped grooves IT, as illustrated in Figure 4. This provides a structure similar to a high tension power line insulator, but the difficulty with such a structure is that it is not as easy and inexpensive to manufacture as the other forms contemplated. Many of the advantages of the present invention may be had even through the peripheral surface of the sleeve or jacket be left smooth and plain, as indicated at la in Figure 6. Obviously, however, this does not attain the maximum increase in the flash-over path along the external surface of the protective sleeve, and is not as water repellent or as resistant to the ac- A variation of the structure shown in Figure 6 is illustrated in Figure 5, where the external surface of the insulating sleeve is provided with a series of longitudinally extending parallel grooves I9.

To enhance the effectiveness of the interconnection of the protective sleeve with the barrel of the plug a suitable cement, designated at C, may be interposed between the inner periphery of the sleeve and the outer periphery of the spark plug barrel 3. Any suitable cement may be used, for example, a chemical setting insulating cement or a cement of the evaporation or absorption type.

The present invention also proposes another specificallydifferent way of eliminating the preexistent flash-over path and of increasing the flash-over path along the outer surface of the spark plug and of protecting the plug against impact and thermal shocks. Thus, as illustrated ning or crimping over a portion 23 of the shell 20. A center electrode 24 extends through an axial opening in the spark plug barrel and is hermetically sealed thereto by suitable cement and is cooperatively interrelated with side or ground electrodes 25. A terminal nut 1 is combined with the outer end of the center electrode in the usual way. In this instance, also, the upper portion of the insulator member is provided with a series of parallel, outwardly projecting, circumferentially extending corrugations 2'I. Rigid, plastic material, designated at 28, is moulded about the protecting portion of the barrel in the manner shown so as to interfit with the corrugations of the barrel, and is itself to be formed with an external series of outwardly projecting corrugations 29 which increase the external flash-over path of the plug and provide a structure which is resistant to the accumulation of moisture and dirt.

The forms of the invention shown in Figures 9 and 10 are similar in construction to that illus-. trated in Figure 8 and differ only in the construction of the insulating barrel of the plug and of the particular construction of the rigid, plastic protecting sleeve. As illustrated in Figure 9, the porcelain barrel of the plug, designated at 30, has a somewhat reduced projection or extension 3|,

. and the size of the moulded plastic sleeve 32 is correspondingly increased. In the form of the invention shown in Figure 10, the porcelain barrel 33 of the plug is still further reduced, and in fact its upper portion terminates within the confines of the metallic shell. In this form of the invention the rigid, plastic protective sleeve 34 has an integral extension 35, which is confined within the upper portion of the metallic shell,

' and is in intimate contact with the upper portion of the insulating barrel.

It will be understood from the foregoing that the present invention makes it practical to provide a plug of the desirable small dimensions,

- assemble it first, and then by the addition of the protective sleeve to increase the outside perimeter of the spark plug barrel insulator so that it is just within the perimeter of the hexagon or other wrench-engaging means formed on the shell. Thus, while the spark plug responds to the limitations of installation space, and to the tions, and a protective sleeve of synthetic rubber having an internal diameter smaller than the external diameter of the outer portion of the barrel and stretched thereover so as to be in tight interfitting contact therewith, said sleeve being substantially coextensive with the exposed surface of said outer portion and said interfitting contact extending for the entire coextensive relationship.

2. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein and having an exposed terminal, cooperable electrodes combined with'the barrel and shell, and a protective sleeve of elastic insulating material, said sleeve being substantially co-extensive with and fitted over the projecting portion of the barrel and having its inner surface in tight surface-to-surface contact with the external surface of the barrel for the entire coextensive relationship, the outer surface of the sleeve increasing the length of the external flash over path of the plug, said sleeve having an opening through which said terminal projects.

3. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein, cooperable electrodes combined with the barrel and shell, the projection portion of said barrel having a series of parallel corrugations extending circumferentially of its outer periphery, and a pre-moulded elastic sleeve of insulating material substantially coextensive with and stretched over the projecting portion of the barrel and maintained in surface-to-surface contact therewith by virtue of its inherent elasticity throughout the entire coextensive relationship.

4. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein, cooperable electrodes combined with the barrel and shell, the projecting portion of said barrel having a series of parallel corrugations extending circumferentially of its outer periphery, and a pre-mouldecl elastic sleeve of insulating material substantially coextensive with and stretched over the projecting portion of the barrel and maintained in surface-to-surface contact therewith by virtue of its inherent elasticity throughout the entire coextensive relationship, said sleeve having a series of parallel, circumferentially extending grooves in its outer peripheryto increase the length of the external fiashover path of the spark plug.

imeter of said engaging means as vertically extended.

6. A spark plug of the character described comprising a metallic shell, a porcelain insulating barrel fitted therein, cooperable electrodes combined with the barrel and shell, and a protective jacket of moulded plastic material combined with the barrel and shell and being in close surface contact therewith throughout their entire overlapping relationship to increase the length of the external flash-over path of the p 7. A spark plug of the characterdescri-bed comprising a metallic shell having wrenchengaging means formed thereon, an insulating barrel fitted therein, and a protective jacket of insulating material fitted over the projecting portion of the barrel to seal off the normal fiashover path along the external surface thereof, the

.jacket having an opening in the top thereof through which said terminal projects.

9. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein and having a vertically projecting exposed terminal, and a protective jacket of 'insulating material fitted over the projecting portion of the barrel to seal off the normal fiashover path along the external surface thereof, the external surface of the jacket having circumferentially extending rounded corrugations, said jacket having an opening in the top thereof through which said terminal projects.

10. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein and having a vertically projecting exposed terminal, and a protective jacket of insulating material fitted over the projecting portion of the barrel to seal off the normal fiashover path along the external surface thereof, the external surface of the jacket having circumferentially extending a series of concave torroidal grooves with sharp edges between the adjacent grooves, said jacket having an opening in the top thereof through which said terminal projects.

11. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein, and a protective jacket of insulating material fitted over the projecting portion of the barrel and coextensive therewith to seal off the normal flash-over path along the external surface thereof and increase the length of the fitted therein, and a protective jacket of insulating material fitted over the projecting portion of the barrel and coextensive therewith to seal off the normal flash-over path along the external surface thereof and increase, the length of the external flash-over path of the plug, the external surface of the jacket having a coating thereon repellent to the accumulation of dirt and moisture.

13. .A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein and having an exposed terminal, and a protective sleeve of electric and heat insulating material, said sleeve being fitted over the projecting portion of the barrel and being coextensive therewith and having its inner surface in tight surface-to-surface contact with the external surface of the barrel throughout the entire coextensive area, said sleeve being of sufficient thickness to protect said barrel against mechanical shock, and an opening in said sleeve through which said terminal projects to protect the plug against thermal shock.

14. A spark plug of the character described comprising a metallic shell, an insulating barrel fitted therein, and a protective sleeve of synthetic rubber, said sleeve being coextensive with the external surface of the projecting portion of the barrel and being in tight surface-to-surface contact therewith throughout the entire coextensive area to seal 01! the normal flash-over path along the external surface of the barrel to increase the length of the external flash-over path of the plug and to protect the plug against impact and thermal shock.

15. A spark plug comprising a metallic shell having an electrode at one end thereof, an insulator fitting in said shell and projecting thereabove enclosing a second electrode operatively associated with said first named electrode and having an upper exposed terminal, and a protective jacket of insulating material completely enclosing and sealably associated with the upper exposed portion of said insulator throughout its length and having an opening in the top thereof through which said terminal projects.

FRED J. BOLL. 

